The invention, in some embodiments, relates to the field of global navigation satellite systems, and more particularly to the field of methods and devices for position determination by receivers of global navigation satellite systems.
The invention, in some embodiments, relates to the field of urban mapping systems, and more particularly to the field of methods for generating a three-dimensional (3-D) representation of an urban area using a receiver of a global navigation satellite system.
Global Navigation Satellite Systems (GNSS) provide autonomous geo-spatial positioning, typically including global coverage. A global navigation satellite system allows an electronic receiver to determine its own position, namely longitude, latitude, and altitude, to within a few meters, using ephemeris data and time signals transmitted by radio from one or more satellites.
A GNSS receiver approximates its position by interpolating the signal from each navigation satellite, and specifically the precise coordinates of the satellite contained in the ephemeris data and the time stamps received from the satellite, into a pseudorange, indicating a region to which the signal from the satellite could travel in the specified time duration. Using at least four such pseudoranges and their associated satellite locations, the GNSS receiver computes its location by intersecting the pseudoranges to obtain a user position. The position is generally provided to a user as a position region, for example a position circle having a calculated center point indicating the most likely position of the receiver, and a radius that indicates an estimated error. Mathematically, four pseudoranges are sufficient to determine the position of the receiver with a reasonable error.
Disregarding topography and terrestrial objects on the Earth's surface, most global navigation satellite systems, such as GPS, GLONASS, and Galileo, have satellite coverage that ensures that a receiver on Earth has simultaneous lines of sight (LOS) to at least four satellites and can therefore accurately determine its position.
In most cases, a strong signal received by a GNSS receiver indicates the existence of a line of sight between the receiver and the satellite. Therefore, a GNSS device operating in an open area typically sorts captured signals according to their strengths, and uses the four strongest signals to calculate its location fairly accurately, typically with an error range within 2-5 meters.
When the line of sight to a number of satellites is blocked, calculated position accuracy may decrease. For example, in many cities, such as downtown Manhattan, the tall buildings or other obstacles form an urban canyon where sky visibility is greatly limited. It is very common for a GNSS device operating in an area of this sort to be surrounded by obstacles which block the line of sight to most, if not all, otherwise available satellites. As mentioned above, at least four strong-enough signals, equivalent to four line-of-sight satellites, are required for accurate position determination. Therefore, a narrowly available sky which allows a GNSS receiver to have a line of sight to less than four satellites, leads to a skewed position computation, or, in the worst case, an inability of the receiver to compute its location. Moreover, objects such as building walls may reflect a signal from a satellite towards the receiver, leading to a mistaken pseudorange calculation.
As a result of the presence of tall buildings and other obstacles, a receiver in urban areas often provides a position circle with a large radius reflecting a greater error in a position calculation. Blocked lines of sight and signal reflections in an urban area may lead to estimated errors in position calculation of a few tens of meters, and in some instances even up to hundreds of meters. In some such cases the GNSS receiver is not able to calculate a position at all.
Several methods for determining whether a satellite has a line of sight (an “LOS satellite”) or does not have a line of sight (an “NLOS satellite”) to the receiver, have been developed, such as those described in U.S. Pat. No. 7,577,445 and U.S. Patent Application Publication No. 2005/0124368.
In addition to methods for determining whether or not a satellite is in line of sight to a receiver, several methods for improving the accuracy of a position calculation in an urban area have been suggested. One such method is Map Matching, which is based on the assumption that the GNSS receiver is located inside a car, which drives at some estimated speed on top of a road with a known path. Thus, the location of the GNSS receiver can be limited by matching a road map of the terrain to the proposed position region, and thereby limiting the possible location of the GNSS receiver to areas of the proposed position region that correspond to roads.
Another such method is known as dead-reckoning. A dead-reckoning navigation system, calculates the current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time, and course. In some systems, the vehicle is equipped with sensors that record the wheel rotation and steering direction which are available as position and speed input for the navigation system receiver. However, one of the disadvantages of dead-reckoning is that new values are calculated using previous values, and therefore errors and uncertainty regarding the calculated values will accumulate over time.